1. Field of the Invention
The present invention relates to a high voltage ceramic capacitor, and in particular, a high voltage ceramic capacitor which is suitable for use in environments requiring halogen resistance. Such capacitors are useful, for example, in an atmosphere containing halogen gas, such as a high voltage ceramic capacitor for an excimer laser apparatus.
2. Description of the Related Art
A high voltage ceramic capacitor is conventionally used for discharge excitation in an excimer laser apparatus. Because it is used in an atmosphere containing halogen gas, it must be resistant to halogen.
A high voltage ceramic capacitor 51 which is designed to be halogen resistant is shown in FIG. 5 and disclosed in Japanese Unexamined Patent Publication No. 8-130158. The high voltage ceramic capacitor 51 includes a capacitor device 52 made of a dielectric ceramic having electrodes 52b, 52c on the top and bottom surfaces 52a thereof. Metal terminals 53, 54 are connected to the electrodes 52b, 52c of the capacitor device 52, respectively.
The capacitor device 52 is arranged in a housing 55. The housing 55 includes a housing body 55a which is open at the top and a lid member 55b which covers the opening. The housing body 55a and the lid member 55b are made of aluminum oxide, which is superior in halogen resistance. One of the metal terminals 53 passes through the lid member 55b, and the other of the metal terminals 54 passes through the bottom of the housing body 55a.
A series of seals are formed to increase the halogen resistance of the capacitor device 57. Seal members 56, 57, made of elastic rings or the like, are arranged between each of the two metal terminals 53, 54 and the housing 55, respectively, to form airtight seals. Another seal member 58, made of an elastic ring, is arranged at the contact surface between the housing body 55a and the lid member 55b to form an airtight seal. Finally, the housing 55 is filled with an epoxy resin 59 to increase halogen resistance.
Because housing 55 made of aluminum oxide and having the above-described airtight sealing construction is used and because an epoxy resin 59 having superior halogen resistance is used to encase the capacitor device 52 and the metal terminals 53, 54 (other than protruding portions of the metal terminals 53, 54), the high voltage ceramic capacitor 51 exhibits an increased halogen resistance.
Another high voltage ceramic capacitor is disclosed in Japanese Unexamined Patent Publication No. 7-230933. This high voltage ceramic capacitor has a configuration in which a capacitor device has electrodes on two main surfaces, the electrodes being connected to metal terminals, and is arranged in an airtight sealed housing made of a ceramic material. In this construction, the tips of the metal terminals protrude outwardly from the airtight sealed housing. A hole is provided with the metal terminals so that the hole passes through from the outside of the housing to the inside thereof. Gases are removed from the inside of the airtight sealed housing through the hole, so that the pressure in the airtight sealed housing is reduced. Then, the airtight sealed housing is filled with an insulating protective material such as an epoxy resin, a silicone resin, a urethane resin, or the like.
In the conventional high voltage ceramic capacitors disclosed in Japanese Unexamined Patent Publication Nos. 8-130158 and 7-230933, when the assembled device, in which a pair of metal terminals are connected to the capacitor device, is arranged in the housing, the resin (epoxy, silicone, urethane, or the like) is filled in as an insulating protective material. However, when epoxy resin is used, the coefficient of linear expansion of the cured epoxy resin is significantly greater than that of the ceramic which makes up the housing. Therefore, cracking may occur in the epoxy resin during curing and shrinking.
When cracks occur in the epoxy resin, the insulation performance of the capacitor deteriorates and lowers the voltage which the capacitor can tolerate. In addition, each component of the high voltage ceramic capacitor reacts with the halogen gas generating secondary gas and dust and polluting the gas atmosphere.
When silicone resin or urethane resin is used instead of epoxy resin, the above-described cracking can be avoided by virtue of its superior elasticity. However, silicone resin and urethane resin are significantly affected by halogen gas, in particular by fluorine gas. When the fluorine gas content increases slightly, the resin becomes fluid.
In the above-described high voltage ceramic capacitor 51, the housing 55 is sealed airtight with seal members 56 to 58 in the form of elastic rings. However, the seal members do not function satisfactorily. Therefore, penetration by halogen gas such as fluorine gas cannot be completely prevented. When silicone resin or urethane resin is used instead of epoxy resin 59, the resin is fluidized leading to separation of the lid member 55b. As a result, the fluidized resin material leaks into the excimer laser apparatus causing deterioration of the gas atmosphere.
Therefore, silicone resin and urethane resin cannot be used in place of epoxy resin 59 in the high voltage ceramic capacitor 51. Thus, as disclosed in Japanese Unexamined Patent Publication No. 7-230933, an airtight sealed housing must be used. However, the airtight sealed housing does not have perfect sealing ability, and therefore halogen gas may enter the housing. When halogen gas enters the housing, the resin is fluidized, causing the same problems described above.
In addition, in the conventional high voltage ceramic capacitor, an insulating protective resin which fills the housing may expand, causing, for example, the lid member of the housing to open during use.